DB review safe-fix: naive-UTC timestamp defaults (drop db.func.now)

DB review found four DateTime columns defaulting to db.func.now() (MySQL
session-timezone wall clock) while the rest of the schema stores naive UTC, so
one schema mixed two clocks and to_dict() labelled the local values UTC with a
'Z' suffix. Switch application.dateadded, computers.installeddate,
knowledgebase.lastupdated (default + onupdate), and slides.uploadeddate to the
module-level naive-UTC _utcnow callable already used elsewhere (apitoken.py).
ORM-side default only - no column-type change, no data migration; affects
new/updated rows.

Targeted tests pass (154); naming + pyflakes green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/slides/models/tvslide.py

@@ -1,8 +1,15 @@
 """Slide model for the TV dashboard / screensaver slideshows."""
 
+from datetime import datetime, timezone
+
 from shopdb.api import db
 
 
+def _utcnow():
+    # naive UTC to match the other DB DateTime columns (stored without tzinfo)
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
 class TvSlide(db.Model):
     """One slide image in a surface's playlist.
 
@@ -17,7 +24,7 @@ class TvSlide(db.Model):
     filename = db.Column(db.String(255), nullable=False)
     sortorder = db.Column(db.Integer, default=0, nullable=False)
     seconds = db.Column(db.Integer, default=0, nullable=False)  # 0 = use default interval
-    uploadeddate = db.Column(db.DateTime, default=db.func.now())
+    uploadeddate = db.Column(db.DateTime, default=_utcnow)
 
     # No DB-level unique on (surface, filename): utf8mb4 pushes that index past
     # MySQL 5.6's 767-byte limit. Upload unique-renames, so dupes can't occur.